The present invention relates to a reflection type liquid crystal display device in which a light-transmitting electrode and a light-reflecting electrode are disposed opposite to each other with a vertically aligned liquid crystal interposed therebetween, a display apparatus using the reflection type liquid crystal display device, and a projection system using the reflection type liquid crystal display device.
In reflection type liquid crystal display devices according to the related art, the thickness of a vertically aligned liquid crystal layer is 3 to 4 μm, and the curve of the liquid crystal transmittance against the drive voltage (hereinafter referred to as V-T curve) has such a characteristic as to rise at a threshold voltage of about 2 V and to reach a maximum at an impressed voltage of 4 to 6 V. This voltage is called saturation voltage. The liquid crystal display devices are driven by inverting the positive/negative voltage on the basis of each frame or field, and, in practice, the devices are driven at a voltage of ±4 to 6 V at maximum.
Where the liquid crystal display devices are used in a three panel type projection system composed of R (RED), G (GREEN) and B (BLUE) three color panels, the saturation voltage differs according to the wavelengths of the colors. This is because it is necessary for an optical path difference called retardation to be one half of the wavelength, for maximizing the transmittance of the liquid crystal (the reflectance in the case of a reflection type liquid crystal cell). The retardation is a quantity expressed by the product of the magnitude of effective refractive index anisotropy and the cell thickness, and the effective refractive index anisotropy increases as the voltage is impressed. As the wavelength is shorter, it suffices for the effective refractive index anisotropy to be smaller, and, hence, for the impressed voltage to be lower. Specifically, the saturation voltage is the lowest for the BLUE wavelength of 450 nm, and is the second lowest for the GREEN wavelength region (550 nm).
In a white lamp used for other systems than projection system, the saturation voltage of the liquid crystal display device is at substantially the same as that for GREEN. However, since the RED wavelength is 650 nm, the saturation voltage is the highest, and, generally, the saturation voltage of the liquid crystal devices in a projection system is higher than the saturation voltage in the case of the while lamp by about 30% to 50%. Therefore, even if the white lamp can be driven at a voltage of ±5 to 6 V, it is highly possible that the saturation voltage for RED in a three panel type projection system might exceed 6 V.
Even under this condition, ordinary Si (silicon) transistors can be driven only at a voltage of 4 to 6 V, so that in the case of a RED liquid crystal display device (panel) it is impossible to display the maximum reflectance intrinsically possessed by the panel. In the three panel type projection, the RGB luminances (brightnesses) must be matched, so that the GREEN and BLUE panels which are intrinsically capable of displaying the maximum reflectance may possibly be used by lowering the luminance thereof.
On the other hand, not only in the case of the reflection type but also in the case of the transmission type, general liquid crystal display devices are asymmetric in electrode structure and shape between two opposed substrates, so that a perfect electrical symmetric relationship is not achieved. When a voltage is impressed on the device for a long time under this condition, the so-called sticking phenomenon occurs in which ions contained in the liquid crystal layer are moved and attached to the electrode on one side. In the liquid crystal display devices of the same kind, the sticking is more liable to occur as the drive voltage is higher; generally, the degree of the sticking is considered to be proportional to the square of the drive voltage. Namely, a lowering in the saturation voltage leads to suppression of the generation of the sticking. From this point of view, a reflection type liquid crystal display device capable of obtaining a high contrast even when driven at a low voltage is disclosed in Japanese Patent Laid-open No. 2003-107482.
However, in a display device using a vertically aligned liquid crystal according to the related art, as the thickness of the liquid crystal layer is reduced, a higher response speed and a higher contrast can be contrived, but the saturation voltage is raised, with the result of an adverse effect on the lowering of the drive voltage. Besides, when a liquid crystal with a high refractive index anisotropy is used while the thickness of the liquid crystal layer is maintained at 3 to 4 μm, the saturation voltage can be lowered, but it is difficult to enhance the response speed, it is impossible to obtain a high contrast and, hence, there arises a limitation in enhancing the performance.